α- and β-Cyclodextrin Rotaxanes of μ-Bis(4-pyridyl)bis[pentacyanoferrate(II)] Complexes

The kinetics and mechanism of the self-assemblies of α- and β-cyclodextrin (CD) [2]rotaxanes, [(NC)5Fe{pyRpy·CD}Fe(CN)5]6-, containing pentacyanoferrate(II)-stoppered 4,4‘-bis(pyridyl) threads pyRpy (R = −CHCH−, −NN−, −CHN−NCH−, and −C(CH3)N−NC(CH3)−) have been investigated in aqueous solution by using visible and 1H NMR spectroscopy. The rotaxanes may be formed rapidly by the addition of the [Fe(CN)5OH2]3- ion to the CD-included pyRpy thread or slowly by the addition of an excess of CD to the dimeric [(NC)5Fe(pyRpy)Fe(CN)5]6- complex. In the latter method, the mechanism involves a rate-determining dissociation of a [Fe(CN)5]3- center to form the monomeric complex, which subsequently includes the coordinated pyRpy in the CD cavity to yield the semirotaxane, which is rapidly recomplexed by the [Fe(CN)5OH2]3- ion, generating the [2]rotaxane. Rate and activation parameters and CD inclusion stability constants have been determined for the ligand substitution reactions involving the formations and dissociation...